Searching for eco-friendly and earth-abundant materials to supersede traditional high-cost bismuth telluride for fabricating wearable devices is of great significance in thermoelectrics. In this work, promising flexible Cu2Se based thin films with high thermoelectric performance is successfully fabricated via a facile co-sputtering method. Experimental results indicate that excess Cu in Cu2Se films leads to the decrease of carrier concentration by suppressing the formation of Cu vacancies and donating electrons, benefiting to achieve high Seebeck coefficient. Moreover, Cu-excess Cu2Se films have highly (0l0) preferred orientation and extra high carrier mobility, maintaining the decent electrical conductivity in the whole measurement temperature range. Combined with the low thermal conductivity, a maximum ZT of 0.42 is obtained at 275 °C from the Cu-excess Cu2Se due to the simultaneous optimization both of electrical and heat transport. Subsequently, a flexible thermoelectric device assembled with high performance Cu2Se films exhibits a maximum power density of 4.28 Wm-2 at a temperature difference of 50 °C, which thermal stability is greatly improved after introducing a molybdenum buffer layer into electrode layer. Therefore, this work demonstrates that rational microstructure manipulations and connection technology improvement can achieve high performance in the flexible thermoelectric device, which possess potential in wearable applications. Display omitted •Flexible Cu2Se based thin films with desired (0l0)-preferred orientation were successfully achieved by optimized growth temperature and composition.•Excess Cu in flexible Cu2Se films contribute to improve electrical and thermal transport properties simultaneously.•A Mo buffer layer introduced into electrode layer enhanced the thermal stability of Cu2Se based flexible thermoelectric device.